1. Field of the Invention
The present invention relates to an image forming method using a photothermographic material. More particularly, the invention relates to an image forming method of forming an image on a double-sided photothermographic material having image forming layers on both surfaces of a support, by X-ray exposure.
2. Description of the Related Art
In recent years, decrease in the amount of processing liquid waste in the field of films for medical imaging has been keenly desired from the viewpoints of environmental protection and economy of space. For this reason, techniques regarding photothermographic materials for medical diagnosis and graphic arts, which can be exposed efficiently by laser image setters or laser imagers and can form clear black-toned images of high resolution and sharpness, is required. According to the photothermographic materials described above, thermal development systems which do not require liquid processing chemicals, are simpler, and do not damage the environment can be supplied to customers.
While similar requirements also exist in the field of general image forming materials, images for medical imaging require a particularly high image quality excellent in sharpness and granularity since fine representation is required, and are characterized in that images of blue-black tones are preferred from the viewpoint of easy diagnosis. At present, various kinds of hard copy systems utilizing dyes or pigments such as ink jet printers and electrophotographic systems have been marketed as general image forming systems, but they are not satisfactory as output systems for medical images.
Thermal image forming systems utilizing organic silver salts are described, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075, as well as in “Thermally Processed Silver Systems” by B. Shely, appearing in “Imaging Processes and Materials”, Neblette, 8th edition, edited by J. Sturge, V. Warlworth, and A. Shepp, page 2, 1996. A photothermographic material generally comprises an image forming layer in which a catalytically active amount of photocatalyst (for example, a silver halide), a reducing agent, a reducible silver salt (for example, an organic silver salt), and if necessary, a toner for controlling the color tone of silver, dispersed in a binder. A photothermographic material forms a black silver image by being heated to a high temperature (for example, 80° C. or higher) after imagewise exposure to cause an oxidation-reduction reaction between a silver halide or a reducible silver salt (functioning as an oxidizing agent) and a reducing agent. The oxidation-reduction reaction is accelerated by the catalytic action of a latent image on the silver halide generated by exposure. As a result, a black silver image is formed in the exposed region. Photothermographic materials have been described in U.S. Pat. No. 2,910,377 and Japanese. Patent Application Publication (JP-B) No. 43-4924, as well as in many other documents. Further, the Fuji Medical Dry Imager FM-DP L is an example of a practical medical image forming system using a photothermographic material that has been marketed.
Attempts have also been made at applying the above-mentioned photothermographic material as photosensitive material for photographing. The “photosensitive material for photographing” as used herein means a photosensitive material on which images are recorded by a one-shot exposure through a lens, rather than by writing the image information by a scanning exposure with a laser beam or the like. Conventionally, photosensitive materials for photographing are generally known in the field of wet developing photosensitive materials, and include films for medical use such as direct or indirect radiography films and mammography films. For example, an X-ray photothermographic material coated on both sides using a blue fluorescent intensifying screen is described in Japanese Patent (JP) No. 3229344, and a photothermographic material using tabular silver iodobromide grains is described in Japanese Patent Application Laid-Open (JP-A) No. 59-142539. As another example, a photothermographic material for medical use containing tabular grains that have a high content of silver chloride and have (100) major faces, and that are coated on both sides of a support, is described in JP-A No. 10-282606. Double-sided coated photothermographic materials are also disclosed in other patent documents (for example, in JP-A Nos. 2000-227642, 2001-22027, 2001-109101, and 2002-90941). However, according to these known examples, although fine particle silver halide grains having a grain size of 0.1 μm or less do not cause further hazing, the sensitivity is very low. These grains are therefore not usable for practical applications in photographing. And conversely, when using silver halide grains having a large grain size, because the remaining silver halide increases the degree of haze and adversely affects the print-out, there is a problem of deterioration of the image quality.
In addition, there has been heretofore no report concerning what kind of characteristics are obtained by thermal development of a photothermographic material having image forming layers on both sides of a support, and what is preferable for an optimum thermally developing method thereof with respect to that of a conventional single-sided coated photothermographic material.
Heretofore, in an image forming apparatus called a laser imager for preparing a print of a visible image from an image measured by a measuring device for medical use such as a CT or an MRI, a photothermographic material having an image forming layer on either side of a support such as a PET film is exposed to a light beam modulated in accordance with image data, and then, the photothermographic material thus exposed is thermally developed by means of a built-in thermal developing section to prepare an image.
FIG. 8 shows a conventional image forming apparatus provided with a thermal developing apparatus (see JP-A No. 11-218894) wherein an image forming apparatus 1 basically comprises a photothermographic material feeding section 3, an image exposing section 5, and a thermally developing section 7 in the order of a conveying direction of a photothermographic material A. The photothermographic material feeding section 3 takes out a sheet of the photothermographic material A from a magazine 9, and feeds the taken out photothermographic material A downstream in the conveying direction. The image exposing section 5 is a region where the photothermographic material A is imagewise exposed by light beam scanning exposure, and which is composed of an exposure unit 11 and a sub-scanning conveying means 13.
The photothermographic material A on which a latent image has been recorded in the image exposing section 5 is conveyed upwards by conveying roller pairs 15 and 17 or the like to the thermal developing section 7. The thermal developing section 7 is a region where the photothermographic material A is heated by the use of a heating drum 19 as a heating means to carry out thermal development, whereby the latent image is made to be a visible image, and which is composed of an endless belt 21, a film peeling claw 23, and support rollers 25a to 25d for supporting the endless belt 21.
The photothermographic material A carried into the thermal developing section 7 is further carried in between the heating drum 19 and the endless belt 21, and it is held between the heating drum 19 and the endless belt 21 by means of rotation of the heating drum 19. The photothermographic material A is conveyed in a state of close contact with the heating drum 19, and it is thermally developed by heat from the heating drum 19, whereby a latent image recorded by exposure becomes a visible image. In this case, only one side of the photothermographic material A is heated by heat from the heating drum 19. When a leading end of the thermally developed photothermographic material A reaches a vicinity of the peeling claw 23, the peeling claw 23 abuts upon the heating drum 19 to enter in between the heating drum 19 and the photothermographic material A, whereby the photothermographic material A is peeled off from the heating drum 19 to be discharged into a discharge tray 27.